Both secondary ion mass spectrometry and transmission electron microscopy have been used to characterize Au-Ni-Ge and Au-Ag-Ge ohmic contacts to modulation-doped GaAs/AlGaAs heterostructures. The time periods for alloying the contact were kept to a minimum (<25 s) in order to obtain controlled diffusion of Ge into the semiconductor. Structural analysis shows that a stable alloy contact is formed after 3 s of alloying at temperatures above the Au-Ge melting point. There were no major structural or compositional changes observed with longer alloying times. The Ge-rich region has a tendency to regrow epitaxially on the GaAs after alloying, as shown by high-resolution electron microscopy.